Re: [PATCH 1/2] media: tegra-cec: Support Tegra186 and Tegra194

[Thierry Reding <thierry.reding@gmail.com>]

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On Tue, Dec 11, 2018 at 10:19:48AM +0100, Hans Verkuil wrote:
> On 12/10/18 9:59 PM, Thierry Reding wrote:
> > On Mon, Dec 10, 2018 at 06:07:10PM +0100, Hans Verkuil wrote:
> >> Hi Thierry,
> >>
> >> On 12/10/18 5:00 PM, Thierry Reding wrote:
> >>> From: Thierry Reding <treding@nvidia.com>
> >>>
> >>> The CEC controller found on Tegra186 and Tegra194 is the same as on
> >>> earlier generations.
> >>
> >> Well... at least for the Tegra186 there is a problem that needs to be addressed first.
> >> No idea if this was solved for the Tegra194, it might be present there as well.
> >>
> >> The Tegra186 hardware connected the CEC lines of both HDMI outputs together. This is
> >> a HW bug, and it means that only one of the two HDMI outputs can use the CEC block.
> > 
> > I don't know where you got that information from, but I can't find any
> > indication of that in the documentation. My understanding is that there
> > is a single CEC block that is completely independent and it is merely a
> > decision of the board designer where to connect it. I'm not aware of any
> > boards that expose more than a single CEC.
> 
> Sorry, my memory was not completely correct.
> 
> The problem is that the 186 can be configured with two HDMI outputs, but it has
> only one CEC block. So CEC can be used for only one of the two. I checked the TRM
> for the Tegra194 and that has up to four HDMI outputs, but still only one CEC
> block.
> 
> And yes, it is the responsibility for the board designer to hook up the CEC pin
> to only one of the outputs, but the TRM never explicitly mentions this and given
> the general lack of knowledge about CEC it wouldn't surprise me at all if there
> will be wrong board designs.
> 
> But be that as it may, the core problem remains: you cannot allow multiple
> HDMI outputs to be connected to the same CEC device.
> 
> However, I now realize that your patches will actually work fine since each
> HDMI connector tries to get a cec notifier for its own HDMI device, but the
> tegra-cec driver will only register a notifier for the HDMI device pointed
> to by the hdmi-phandle property. So only one of the HDMI devices will actually
> get a working CEC.
> 
> Although if board designers mess this up and connect multiple CEC lines to
> the same CEC pin, this would still break, but there is nothing that can be
> done about that. I still believe the TRM should have made this clear since
> it is not obvious. Even better would be to have the same number of CEC blocks
> as there are configurable HDMI outputs. Typically, if you support CEC on one
> HDMI output, you want to support it for all. And today that's not possible
> without adding external CEC devices (as we - Cisco - do).

I wasn't aware that anyone was using a Tegra with support for multiple
HDMI outputs. Do you have a contact that you can forward this kind of
request to? It certainly sounds like something that would be useful to
add in future chips if there's a customer need.

I can also forward this internally, but I expect it to have more weight
coming directly from Cisco. =)

> Apologies for the confusion, I should never send emails after 5pm :-)

No worries.

Thierry

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